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LM2672: Input capacitor selection of LM2672

shiwei luan
Intellectual 750 points
Part Number: LM2672

www.ti.com/.../lm2672.pdf

From datasheet Page 18, it says "Use caution when using ceramic capacitors for input bypassing, because it may cause severe ringing at the VIN pin."

Why ceramic capacitor may cause severe rings compared to Aluminum or Tantalum capacitors?

Thank you.

  • 0
    TI__Expert 4100 points
    Hello,
    The reason for this warning lies in the nature of ceramic capacitors and their inherent performance. Ceramic capacitors have inherently low ESR and in combination with long cables/traces can cause ringing on input. For that reason it is recommended to use combination of ceramic and electrolytic capacitors for input filtering. Please review application note below for details regarding input capacitor selection for DCDC regulators.
    www.ti.com/.../slyt670.pdf

    Regards
    Brani
  • 0 in reply to Brani Dubocanin
    Intellectual 750 points

    If I understand correctly, the low ESR of ceramic capacitor is not good because the ceramic capacitor and the parasitic inductance can form a very good LC structure that can then cause ringing, so electrolytic capacitor with high ESR is better to suppress the ringing?

    Thank you.

  • 0 in reply to shiwei luan
    TI__Genius 11085 points
    Ceramic caps are very important in bypassing high frequency swithcing noises at the input side. It is good to have a very small value ceramic cap (0.1uF to 1uF) at the closest spot to VIN ang GND pins. Then 10uF to 22uF ceramic caps next to the small ceramic.

    If the circuit locates far from the source, or parasitic inductance exists from the source to the ceramic caps, then you need to add a higher ESR cap before the ceramics (closer to the source) to damp the oscillation.